The present invention relates to a charge-coupled-device (CCD) camera system for detecting near-infrared (NIR) wavelengths, involving (a) a color CCD camera having a multitude of channels including red and near-infrared responsive channels, green responsive channels, and blue responsive channels, and (b) filter means which allow near-infrared light to pass and which block red light; wherein the CCD camera system does not include filter means which block near-infrared light.
The reflectance spectrum of vegetation (FIG. 1) shows that vegetation has a strong absorption at visible wavelengths (400-700 nm) and has a strong reflection (and transmission) at near-infrared (NIR) wavelengths (700-1100 nm). The strong absorption in the visible is due to various leaf pigments, principally chlorophyll, and the strong reflection is due to leaf mesophyll cell structure (Gates, D. M., et al., Applied Optics, 4: 11-20 (1965); Knipling, E. B., Remote Sensing of Environment, 1: 155-159 (1970)). Chlorophyll is the key pigment harvesting light energy for photosynthesis and variations in chlorophyll are used to detect various stresses, such as plant nutrient deficiency. There is scientific debate as to which band in the visible is best, green (500-600 nm) or red (600-700 nm), for detection of chlorophyll concentration. However, the NIR band has been found to be critical for detection of vegetation biomass, leaf area index, cover, and health, so that all satellite sensors for remote sensing of natural resources have a primary band in the near-infrared.
Vegetation monitoring using color-infrared film photography was first established by Colewell (Colewell, R. N., Hilgardia, 26: 223-286 (1956)). Film that is sensitive to green, red and NIR is exposed and then developed so that the green film layer is displayed as blue, the red film layer is displayed as green, and the NIR film layer is displayed as red in order to be visually inspected by the human eye (FIG. 2A). Because of vegetation is highly reflective in the NIR and absorptive in the visible (FIG. 1), color-infrared photographs show vegetation as red, hence the term “false-color photographs.”
Digital photography uses silicon-diode charge-coupled detectors in cameras where the silicon diodes have a spectral sensitivity from about 350 nm to about 1100 nm (Parr, A. C., NIST Technical Note 1421, National Institute of Standards and Technology, Gaithersburg, Md. (1996)). Most digital cameras use a Bayer pattern array of filters (U.S. Pat. No. 3,971,065) over an array of detector elements to obtain red, green and blue channels for a digital image pixel (FIG. 2B). All of the Bayer filters transmit at least some NIR light to either the blue, green or red channels, so almost all of the commercially-available digital cameras have an internal NIR-blocking filter. The internal NIR-blocking filter is removable thus allowing reflected NIR photons from vegetation to reach the detector elements.
Certain digital cameras (for example Kodak DCS cameras) have Bayer filters where the red, green and blue Bayer-pattern filters all transmit NIR light (FIG. 2C). When the internal NIR-blocking filter is removed, Zigadlo et al. (U.S. Pat. No. 6,292,212) found that placing a blue-blocking filter in front of the lens allowed the blue channel to record the NIR photons reflected from vegetation. With elaborate calibration, the contribution of NIR photons to the green and red channels are calculated and are subtracted from the signal. Therefore, after extensive post-processing, the raw digital camera image is converted into a red, green and NIR false-color image (FIG. 2C). Currently, the few color-infrared digital cameras that are commercially available are based on the patent of Zigadlo et al.
For many commercially-available digital cameras, only the red Bayer-pattern filters transmit significant amounts of NIR photons, so the method of Zigadlo et al. cannot be applied. In addition, post-processing of each raw image to obtain a false-color image presents a significant extra workload that becomes a burden when processing large numbers of images. Furthermore, the calibration of the digital camera will change with temperature and with time, so the corrections for the green and red bands will change.
The present invention allows color-infrared digital photographs to be obtained at lower cost and without post-processing, and where a NIR, green and blue digital image is obtained instead of a NIR, red and green image as in Zigadlo et al.